Controlled explosions at apartment

Authorities have begun disarming trip wires and explosive devices "set up to kill" inside the apartment of the suspect in the Colorado cinema shooting, hoping to find clues to his motive without destroying key evidence in a blast.

James Holmes is the suspect in the murder of 12 people in a shooting at a cinema in Colorado, the US (AP)

Sgt Cassidy Carlson, centre, speaks to the media on the latest developments in Colorado (AP)

Tom Sullivan embraces family members after searching in vain for his son Alex who celebrated his birthday by going to see The Dark Knight Rises (AP)

Authorities have begun disarming trip wires and explosive devices "set up to kill" inside the apartment of the suspect in the Colorado cinema shooting, hoping to find clues to his motive without destroying key evidence in a blast.

Federal authorities detonated one small explosive and disarmed another inside James Holmes's suburban Denver apartment, but several other explosive devices remain, said Aurora police Sgt Cassidee Carlson.

Meanwhile, makeshift memorials have sprung up near the cinema for the 12 victims, who included a US navy sailor, an aspiring sportscaster, a man celebrating his 27th birthday and a six-year-old-girl, after police grimly went door to door with a list of those killed in the worst mass shooting in recent US history.

Holmes, 24, was arrested early on Friday outside the Aurora cinema after witnesses say he unleashed gunfire and gas canisters on a crowd of film-goers watching the midnight showing of the new Batman movie The Dark Knight Rises.

On Saturday, law enforcement officials began trying to disarm up to 30 devices in Holmes's booby-trapped apartment, and set off a small explosion to detonate one device.

The devices were "set up to kill that person and that could have been a police officer executing a search warrant," Carlson said. Police planned an intricate procedure to disarm the possible weapons without destroying evidence that could be in the apartment.

"We don't want to lose evidential value," Carlson said.

Federal officials said in a bulletin obtained by The Associated Press that they still hadn't determined a motive for the suspect as families grieved and others waited at hospitals, where seven of the wounded remain in critical condition with some injuries that could be permanent.

In his Saturday radio address, President Barack Obama urged Americans to pray "for the victims of this terrible tragedy, for the people who knew them and loved them, for those who are still struggling to recover."